feat(tiered_pricing): 🟢 make test pass

Author: oflorez1381

exercises/tiered_pricing/solutions/oflorez/src/main/java/tv/codely/checkout/TieredPricing.java

@@ -3,6 +3,9 @@
 public class TieredPricing {
 
     public int calculateTotalPrice(int amount_subscriptions) {
+
+        if (amount_subscriptions <= 0) throw new IllegalArgumentException();
+
         return Tier.getTotalPrice(amount_subscriptions);
     }
 }